This invention relates to the rheological characterization of thermoplastic resinous polymers. More particularly, the invention relates to the continuous and simultaneous measurement of polymer viscosity over a wide range of shear rates, whereby the extrusion performance of a polymer can be predicted.
For predicting polymer processability, rheological characterization over a wide range of shear rates is necessary. This characterization generally takes the form of a flow curve, which is a logarithmic plot of shear stress or viscosity against shear rate. Such commonly used tests as melt index (ASTM D-1238) are inadequate for this purpose, since they define only one point on the flow curve, and that point is in a shear-rate range well below normal processing shear rates. Even a two-point measurement is not adequate, since flow curves for non-Newtonian fluids exhibit significant changes in slope over any appreciable range of shear rates.